A conventional boiling water reactor is shown in FIG. 1. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14. The feedwater flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and a core shroud 18. The core shroud 18 is a stainless steel cylinder which surrounds the nuclear fuel core 20, which is made up of a plurality of fuel bundle assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 1). Each array of fuel bundle assemblies is supported at the top by a top guide 24 and at the bottom by a core plate 26.
The water flows through downcomer annulus 16 to the core lower plenum 25. The water subsequently enters the fuel assemblies 22, wherein a boiling boundary layer is established. A mixture of water and steam enters the core upper plenum under the shroud head 28. The steam-water mixture flows through standpipes 30 and enters steam separators 32.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 34 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 36 (only one of which is shown) via recirculation water inlets 38. The jet pump assemblies are circumferentially distributed around the core shroud 18.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
In particular, stress corrosion cracking has been found in the vertical seam welds or heat affected zones thereof in the core shroud 18. This diminishes the structural integrity of the shroud, which vertically and horizontally supports core top guide 24 and shroud head 28. Thus, there is a need for a method and an apparatus for repairing a shroud which has cracks in or near the vertical seam welds.